1. Field of the Invention
This invention relates to lubricating oil compositions utilizable as a lubricant in various industrial fields and more particularly to lubricating oil compositions which have good oxidation stability and in which sludges are difficult to form.
2. Prior Art
As is well known in the art, mineral oils having a low aromatic content and synthetic oils, e.g. poly-alphaolefin oils, which are free of aromatic rings in the structural units, are poor in oxidation stability. When antioxidants are added to the mineral oils or synthetic oils, the resulting lubricating oil compositions exhibit high oil stability. However, a problem involved in these compositions is that the solubility of antioxidants in the oils is low. Antioxidants invariably undergo a degradation when they are oxidized use, and the degraded substances formed by oxidation are left as a sludge.
For the development of long-life lubricating oils, importance should be placed on how to balance this degree of formation of sludge and their service life under oxidizing conditions.
Although phenyl-alpha-naphthylamine is known as a good antioxidant, it has the drawback that the solubility is low and that the solubility of the substances, produced by oxidation dimers, trimers and polymers of these substances is lower. Accordingly, even when high oxidation stability is required, the naphthylamine which has high oxidation stability cannot be used in large amounts in order to improve the oxidation stability of lubricating oils.
In order to improve the solubility of phenyl-alpha-naphthylamine, British Patent No. 1,552,720 discloses a process of alkylating the phenyl group with a propylene trimer. Alternatively, U.S. Pat. No. 3,696,851 discloses a process of alkylating with a propylene dimer or a dimer or trimer of isobutylene. The N-alkylphenyl-alpha-naphthylamines obtained by alkylation of the phenyl group with a propylene or isobutylene dimer or trimer exhibit an improved solubility in oils but have the disadvantage that the solubility of the substances produced by oxidation is still low.